In an optical communication system, intensity modulated signal light is obtained by changing a current injected into a semiconductor laser. The intensity modulated signal light is propagated through an optical fiber of a transmission line to be received by a light receiving apparatus utilizing a photoelectric conversion element such as a PIN diode device, etc. This system which is defined "an intensity modulated - direct detection system" is mainly utilized in the optical communication system. In this optical communication system, it is known that quality-degradation of transmitted light occurs due to the influence of optical fiber dispersion, when light transmission is carried out at a transmission speed of more than gigabit by using signal light of 1.5 .mu.m wavelength band, at which the loss of an optical fiber becomes the lowest value. This is explained, for instance, in the report of "Long-distance Gigabit-Range Optical Fiber Transmission Experiments Employing DFB-LD's and InGaAs-APD's" described in pages of 1488 to 1497 of "IEEE, Journal of Lightwave Technology, Vol. LT-5, No.10" by M. Shikada et al.
On the other hand, in an optical heterodyne communication system, in which information is regenerated from beat obtained in accordance with the mixture of oscillation light at an optical receiving apparatus receiving the information carried on frequency, phase or amplitude of light from an optical transmitting apparatus, the influence of optical fiber dispersion is low as compared to an optical communication system using intensity modulation-direct detection, because there is no influence of spectrum extension due to chirping which occurs at the time of direct modulation of a semiconductor laser. However, it is reported that degradation occurs in a light transmission of a ultra high speed and a long distance, for instance, as explained in "Chromatic Dispersion Equalization in an 8 Gb/s 202 km CPSK Transmission Experiment" of 17th Conference on Integrated Optics and Optical Fiber Communication, Post-deadline Papers 20 PDA-13" by N. Takachio et al.
In addition, research of an optical amplifier and a direct amplification repeating system using the optical amplifiers has been carried out intensively recently for instance, as explained in "516 km 2.5 Gb/s Optical fiber Transmission Experiment using 10 Semiconductor Laser Amplifiers and Measurement of Jitter Accumulation" of 17th Conference on Integrated Optics and Optical Fiber communication, Post-deadline Papers 20 PDA-9" by S. Yamamoto et al.
In such direct amplification repeating systems, it is expected that a light transmission of a ultra long distance will be realized, because the transmission distance can be extended by compensating the loss of signal light.
As understood from the above, signal light is subject to the loss of a power and the distortion of waveform due to the influence of optical fiber dispersion. Thus, a signal light transmission distance is limited by the power loss and the influence of dispersion. In a high speed transmission of more than several Gb/s, the distance is mainly limited by the influence of dispersion, prior to the consideration of the limitation due to the power loss, because the extension of spectrum exists in signal light due to modulation thereof to increase the influence of dispersion. On the other hand, in a ultra long distance transmission using optical amplifiers as amplifying repeaters, the distance is limited by the influence of dispersion, although the limitation of a power loss is compensated by the optical amplifiers.
Dispersion of an optical fiber is caused by the difference of times, in which signal lights having different frequencies supplied to an input terminal of the optical fiber are propagated therethrough. Therefore, if the extension of spectrum exists in the signal lights, waveforms of the signal lights are distorted through the transmission thereof. For instance, when signal lights of 1.5 .mu.m band are propagated through a zero-dispersion optical fiber of 1.3 .mu.m band, a short wavelength component of the signal lights (a high frequency signal component) has a high transmission speed, while a long wavelength component of the signal light (a low frequency signal component) has a low transmission speed. Therefore, the high frequency signal component is converged at the front portion of a transmitted pulse, and the low frequency signal component is converged at the rear portion thereof. As a result, the transmitted pulse is subject to the distortion of waveform, so that the discrimination of symbols such as mark and space tends to be impossible.
A method for compensating the waveform distorsion resulted from the aforementioned dispersion is proposed by "Dispersion Compensation by Active Predistorted Signal Synthesis" described on pages 800 to 805 of "IEEE, Journal of Lightwave Technology, Vol. LT-3, No. 4" by T.L. Koch et al. In this method, light emitted from a semiconductor laser is modulated in frequency, and the frequency-modulated light is modulated in intensity by an ideal external modulator of LiNbO.sub.3, so that signal light is predistorted to be low in frequency at the front portion of one pulse and high in frequency at the rear portion thereof. Then, the predistorted signal light is propagated through an optical fiber. Consequently, it is reported that an extended amount of a pulse width is decreased in the transmitted signal light pulse.
The ideal external modulator is a modulator which applies only intensity-modulation to input light. Such an ideal external modulation is realized by an external modulator of LiNbO.sub.3. In fact, however, this LiNbO.sub.3 modulator provides undesirable phase-modulation simultaneously with intensity-modulation due to the existence of an asymmetrical electrode structure. Even in a semiconductor absorption type modulator which is expected to be integrated with a semiconductor laser, undesirable phase-modulation is generated together with intensity modulation, as explained in "Frequency Chirping in External Modulators" described on pages 87 to 93 of "IEEE, Journal of Lightwave Technology, Vol. 6, No. 1, Jan. 1988" by F. Koyama et al. In accordance with the undesirable phase modulation, one of two phenomenons, in which an extended amount of a transmitted pulse width is increased or decreased is found. In the former LiNbO.sub.3 modulator, the extended amount can be decreased, as explained in "10 Gb/s Transmission in Large-Dispersion Fiber Using a Ti:LiNbO.sub.3 Mach-Zehnder Modulator" described on pages 208 and 209 of "Technical digest of 17th International Conference on Integrated Optics and Optical Fiber Communication, Vol. 3, Kobe, Japan 1989" by T. Okiyama et al. The latter semiconductor modulator is known to increase the extended amount.
A method, in which output light of a semiconductor laser is modulated in frequency, and the frequency-modulated light is modulated in intensity by an ideal LiNbO.sub.3 external modulator, so that the modulated light is propagated through an optical fiber, is defined "prechirp method" hereinafter, and the transmitting wave is defined "prechirp wave" hereinafter.
However, this prechirp method has a disadvantage in that it is difficult to be adapted to an NRZ format, in which a pulse width is not constant to be more than one time slot, although it can be adapted to codes such as a RZ format, in which a pulse width is constant to be less than one time slot, because frequency-shift of one direction is carried out within one pulse. In this connection, a RZ format has a disadvantage in that a signal band is extended, and the influence of dispersion is large, because the RZ format utilizes a signal having a narrow pulse width as compared to the NRZ format.
The prechirp method has a further disadvantage in that the size is large, and the cost is high, when a LiNbO.sub.3 modulator is used, because the LiNbO.sub.3 modulator is large in size and high in cost as compared to a semiconductor electro-absorption modulator which can be integrated with a semiconductor laser to provide a small-sized transmitting apparatus.
The prechirp method has a still further disadvantage in that an extended amount of a pulse width is large in a transmitted pulse due to the generation of undesirable phase-modulation, when an external modulator having a property of generating phase-modulation simultaneously with intensity modulation is used.